Long-term behaviour of coir-fibre-reinforced soils

Despite their promising results as reinforcement material, the use of natural fibres often raises concerns about long-term performance and aspects related to biodegradation. This study carried out a comprehensive triaxial testing programme involving treated and non-treated and aged and non-aged specimens to elucidate some aspects of the mechanical behaviour of coir-fibre-reinforced soil. Considering the response of the non-aged composites in triaxial tests, the fibres increased the soil shear strength regardless of the applied surface treatment. However, hornification (cohesion and friction angle increments of 36 kPa and 5°, respectively) was the only treatment that improved the strength of composites compared with that of untreated natural fibres (cohesion and friction angle increments of 26 kPa and 5°, respectively). After 5 months, all the composites except those with coupled treatment (sodium hydroxide (NaOH) + hornification) had higher shear strength than natural soil and strengths similar to those of composites tested just after compaction. Hornificated fibre composites also had the highest shear strength among aged composite samples. The fibre tensile strength behaviour was consistent with mechanical response of the composites, and treated fibres helped prevent superficial soil cracking due to weathering exposure. The results encourage the use of coir-fibre-reinforced soils, mainly in temporary earthworks such as landfill cover layers.